This invention concerns a process for recovering and purifying certain herbicidal phenoxybenzoic acid compounds and derivatives thereof. Such compounds are highly effective herbicides for the post emergent control of a variety of weeds such as certain broadleafs associated with soybeans and can be prepared via a four-step synthesis as follows: ##STR1## X.sub.1, X.sub.2 and X.sub.3 each is H, F, Cl, Br, CF.sub.3, OCF.sub.2 CHZ.sub.2 (Z=Cl, F or Br), OCF.sub.3, CN, CO.sub.2 R (R=lower alkyl), -C.sub.6 H.sub.5, O-alkyl, NO.sub.2 or SO.sub.2 (lower) alkyl and X.sub.4 is F, Cl or Br provided that at least one of X.sub.1, X.sub.2 and X.sub.3 is other than H. ##STR2##
The carboxyl group in the product compounds can be made to undergo transformation to a variety of other groups, including salts, employing conventional procedures. The foregoing four-step synthesis is particularly advantageous for the production of acifluorfen, i.e., 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid, which in the form of its sodium salt is commercially available as the product Tackle (Mobil Oil Corporation). Acifluorfen possesses the structure ##STR3##
However, in practice, the coupling reaction of Step 2 and the nitration of Step 4 each produces several weight percent of isomers and other undesired compounds which are carried through to the end of the process. Thus, for example, the following undesirable isomers and by-product impurities have been found in association with commercial acifluorfen:
______________________________________ ISOMERS AND OTHER COMPOUNDS ASSOCIATED WITH COMMERCIAL ACIFLUOREN Weight Isomer/Other Impurity Percent ______________________________________ 3-[2-chloro-5-(trifluoromethyl) phenoxy] benzoic acid &lt;0.5 3-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid &lt;0.5 3-[2,6-dichloro-4-(trifluoromethyl) phenoxy] benzoic acid* &lt;0.5 Nitro 4-[2-chloro-4-(trifluoromethyl) phenoxy] benzene &lt;0.5 Nitro 4-[2-chloro-4-(trifluoromethyl) phenoxy] toluene &lt;0.5 2-Bromo-5-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid* &lt;0.5 4-Nitro-5-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid &lt;3.0 2-Nitro-5-[2-chloro-5-(trifluoromethyl) phenoxy] benzoic acid &lt;3.0 2-Nitro-3-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid &lt;16.0 2-Nitro-3-[2,6-dichloro-4-(trifluoromethyl) phenoxy] benzoic acid* &lt;0.5 2-Nitro-5-[2,6-dichloro-4-(trifluoromethyl) phenoxy] benzoic acid* &lt;0.5 2-Nitro-5-[2-chloro-4-(trifluoromethyl)-6-nitrophenoxy] benzoic acid* 2-Nitro-4t-butyl-5-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid* &lt;0.5 2,4-Dinitro-5-[2-chloro-4-(trifluoromethyl) phenoxy] benzoic acid &lt;0.5 ______________________________________ *tentative identification
Accordingly, a need exists for a procedure for recovering and purifying phenoxybenzoic acid derivatives in general and acifluorfen in particular.